The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Generally speaking, techniques for positioning computing devices rely on high-altitude signal sources as Global Positioning System (GPS) satellites or pseudo-satellites, terrestrial signal sources such as 802.11 (or Wi-Fi™) access points or cellular base stations, or proximity sensing. For example, a portable computing device such as a smartphone can receive GPS signals from several satellites and determine its location on the surface of the Earth based on these signals. Similarly, a portable computing device can determine its location using a signal from an 802.11 access point or, for greater accuracy, signals from several 802.11 access points.
However, GPS satellites generally are obscured inside buildings, tunnels, underground, etc., causing multipath errors or failure to generate a positioning fix at all. Moreover, GPS positioning techniques generally are not optimized for determining elevation, and, as a result, the vertical component of a typical GPS positioning fix is relatively inaccurate even when GPS signals are not occluded. To the extent that a barometer can be used to determine elevation, barometer measurements often also are highly inaccurate.